MINOR: fix failing ZooKeeper system tests (apache#10297)

ZooKeeper-related system tests in zookeeper_security_upgrade_test.py and
zookeeper_tls_test.py broke due to apache#10199. That patch changed the logic of
SecurityConfig.enabled_sasl_mechanisms() to only add the inter-broker SASL
mechanism when the inter-broker protocol was SASL_{PLAINTEXT,SSL}. The
inter-broker protocol is left to default to PLAINTEXT for the SecurityConfig
instance associated with Zookeeper since that value doesn't apply to ZooKeeper,
so the default inter-broker SASL mechanism of GSSAPI was not being added into
the set returned by enabled_sasl_mechanisms(). This is actually correct --
GSSAPI shouldn't be added since inter-broker communication is a Kafka concept
and doesn't apply to ZooKeeper. GSSAPI should be added when ZooKeeper uses it,
though -- which is the case in these tests. So the prior patch referred to
above uncovered a bug: we were relying on the default inter-broker SASL
mechanism to signal that Kerberos was being used by ZooKeeper even though the
inter-broker protocol has nothing to do with that determination in such cases.
This patch explicitly includes GSSAPI in the list of enabled SASL mechanisms
when SASL is enabled for use by ZooKeeper.

Reviewers: Colin P. McCabe <[email protected]>

tests/kafkatest/services/security/security_config.py

@@ -372,6 +372,9 @@ def interbroker_sasl_mechanism(self):
 
     @property
     def enabled_sasl_mechanisms(self):
+        """
+        :return: all the SASL mechanisms in use, including for brokers, clients, controllers, and ZooKeeper
+        """
         sasl_mechanisms = []
         if self.is_sasl(self.security_protocol):
             # .csv is supported so be sure to account for that possibility
@@ -382,6 +385,8 @@ def enabled_sasl_mechanisms(self):
             sasl_mechanisms += list(self.serves_raft_sasl)
         if self.uses_raft_sasl:
             sasl_mechanisms += list(self.uses_raft_sasl)
+        if self.zk_sasl:
+            sasl_mechanisms += [SecurityConfig.SASL_MECHANISM_GSSAPI]
         return set(sasl_mechanisms)
 
     @property